1. Field of the Invention
This invention relates to an optical head for use in an optical information recording-reproducing apparatus such as a magneto-optical disk apparatus, and particularly to an optical head using a light source having a plurality of light emitting points. The optical head of the present invention can be applied, as an optical head for effecting at least two of the functions of erasing, recording and reproduction of information at a time.
2. Related Background Art
In the field of optical information recording and reproduction, there have heretofore been proposed optical heads capable of effecting erasing and recording or recording and reproduction at a time by the use of a plurality of beams. As one of such proposals, mention may be made of the technique described in Japanese Laid-Open Patent Application No. 58-220247. The outward optical system of it is such as shown in the block diagram of FIG. 1 of the accompanying drawings. In FIG. 1, two laser beams emitted from a monolithic semiconductor laser array 5 having two light emitting points are imaged as light spots for recording and reproduction on a magneto-optical disk 12 which is an optical information recording medium through a collimator lens 8, a beam splitter 23 and an objective lens 11, and recording and reproduction can be accomplished at the same time by these two spots.
In the monolithic semiconductor laser array as used in the above-described prior-art optical head, the spacing between the light emitting points thereof is generally of the order of 100 .mu.m or more in order to avoid thermal interference and electrical interference between the light emitting points. In the optical head of the construction as shown in FIG. 1, the spacing L between the spots on the magneto-optical disk 12 is EQU L=d.multidot.(f.sub.0 /f.sub.col),
where d is the spacing between the light emitting points, f.sub.col is the focal length of the collimator lens and f.sub.0 is the focal length of the objective lens 11. Usually, in an optical head capable of recording and erasing, the numerical aperture of the collimator lens is 0.2 or greater and the numerical aperture of the objective lens is of the order of 0.52-0.55 in order to effectively utilize the light from a semiconductor laser, and when the effective diameter of these lenses is of the order of 4 mm.phi., f.sub.col .ltoreq.10 mm for f.sub.0 =3.8-4 mm. Accordingly, from the above equation, the spacing L between the spots is 38 .mu.m or greater.
Now, information tracks on the magneto-optical disk create eccentricity by the rotation of the disk. The amount of eccentricity of the tracks is usually 100 .mu.m at greatest. Therefore, in the optical head as described above, relative track deviation occurs between the two spots. This will now be described with reference to FIG. 2 of the accompanying drawings. The reference numeral 1 designates a track of radius a having a point 2 as its center. Two spots 3 and 4 are spaced apart by a distance L from each other, and both of them are on the track 1. When the track becomes eccentric, for example, when the track 1 is rotated by an angle .theta. about the spot 3 and moves to 1', the center of the track moves to 2' and the position on the track which corresponds to the spot 4 moves to 4'. The spacing between the position 2 and the position 2' corresponds to the amount of eccentricity W, and the spacing between the position 4 and the position 4' is the amount of relative track deviation .DELTA.T. Approximately, W.perspectiveto.a.multidot..theta. and .DELTA.T .perspectiveto.L.multidot..theta. and accordingly, .DELTA.T .perspectiveto.L.multidot.W/a. If for example, a=30 mm and W=100 .mu.m, when L=38 mm, .DELTA.T.perspectiveto.0.13 .mu.m and thus, in the prior-art optical head as described above, there is caused relative track deviation of 0.13 .mu.m or more.
So, in Japanese Laid-Open Patent Application No. 1-70936, in order to make the amount of relative track deviation .DELTA.T small, it is proposed to dispose two independent semiconductor chips in opposed relationship with each other to construct a hybrid array, and shortening the spacing d between light emitting points while avoiding thermal interference and electrical interference between the light emitting points to thereby shorten the spacing L between spots. The outward optical system of it is shown in the block diagram of FIG. 3 of the accompanying drawings. Semiconductor laser chips 101 and 102 are disposed on a mount member 25 in opposed relationship with each other. Two laser beams emitted from the semiconductor laser chips 101 and 102 are collimated by a collimator lens 8, are transmitted through a half wavelength plate 24 and a beam splitter 23 along substantially the same optical path, are reflected by a mirror 10 and are stopped down as spots 3 and 4 for recording and reproduction, respectively, on a magneto-optical disk 12 by an objective lens 11. Since the light source is of an array construction comprising semiconductor laser chips opposed to each other, the laser beams emitted from the semiconductor laser chips become s-polarized lights relative to the beam splitter 23. Because of the characteristic of the beam splitter 23, s-polarized lights are inconvenient and therefore, the s-polarized lights are converted into p-polarized lights by the half wavelength plate 24. In this example of the prior art, the spacing d between the light emitting points is 27 .mu.m, and when the above-mentioned values of the focal length of the collimator lens and the focal length of the objective lens are adopted, the spacing L between the spots is of the order of 10.3 .mu.m, and the corresponding amount of relative track deviation .DELTA.T is 0.035 .mu.m when eccentricity is 100 82 m.
Generally, in a magneto-optical disk, the allowable amount of track deviation is of the order of 0.1 .mu.m. Further, in the foregoing description, it has been assumed that there is no track deviation at the position of the spot 3, but actually, at this position as well, track deviation occurs due to a servo follow-up error or the like. Accordingly, distributed to the spot 3 and the spot 4 at a square mean, the order of 0.07 .mu.m or less is desired as the amount of relative track deviation .DELTA.T. That is, the order of 21 .mu.m or less is desired as the spacing L between the spots.
Now, in the above-described example of the prior art using a hybrid semiconductor laser array, the spacing between the light emitting points is made small by the construction in which the laser chips are opposed to each other, thereby realizing an amount of relative track deviation .DELTA.T of 0.07 .mu.m or less, but the construction in which the laser chips are opposed to each other requires a half wavelength plate, thus resulting in an increase in the number of parts. Further, the fact of being hybrid leads to a construction in which the positions of the two light emitting points are held by the mount member and the spacing between the light emitting points is liable to change and therefore, as compared with the case of being monolithic, there is a problem in respect of the reliability in the light source portion.